The mortality rate associated with cancer motivates our continued development and assessment of new treatment methods such as high intensity focused ultrasound (FUS) ablation. With MR imaging guidance, FUS thermal therapy represents a promising cancer treatment method. MRI plays an important role in FUS treatment, providing temperature sensitive images for treatment monitoring as well as high resolution post-ablation T2 or contrast enhanced T1 weighted images for treatment verification. Unfortunately, these imaging methods are incomplete. Tissue necrosis or cell death is inferred from the MR temperature mapping during treatment and MR imaging of the tissue transient effects after the treatment is complete. The success of FUS thermal therapy is hindered by the indirect link between the MR image contrast and indicators of tissue ablation. The goal of this proposal is to develop MR elasticity imaging to measure FUS excited tissue displacement as a means to directly monitor FUS thermal ablation during each treatment; and verify after all treatments are complete that the targeted tissue volume is entirely ablated. The specific aims in this proposal_ are (1) to develop and test a method using MRI based tissue stiffness measures as a real time monitor for cellular changes indicative of ablation at the focus during FUS treatment; and (2) to develop and test a method to use MRI based tissue stiffness imaging to verify that tissue within a targeted treatment volume is fully ablated after FUS treatment is complete. Should the patient move during treatment, aim 2 may represent the only method available to realign the treatment with the new position and to re-establish the thermal treatment without starting over. The proposed methods are implemented on a standard MRI imager with focused ultrasound therapy equipment attachment. This research will provide data using the viscoelastic properties of thermally treated tissue to improve the MRI guided FUS thermal treatment of cancer through reliable monitoring and immediate assessment of tissue coagulation.